1. Field of the Invention
This invention relates to electroencephalographic (EEG) probe lead wiring, and more particularly to a lead wire for an EEG probe which minimizes the noise introduced into an EEG measuring system due to the wiring between an EEG probe and an amplifier of an evoked potential autorefractometry system.
2. Description of the Related Art
In EEG systems, signals detected by the EEG probes are typically amplified by one million. Amplifiers for this purpose normally include very expensive low pass filters to filter out the noise picked up by the EEG probe and wiring between the probe and the amplifier. FIG. 1 illustrates a conventional EEG measurement system. In FIG. 1, noise due to lead wires 10 and 15, respectively connected between the EEG probes 20 and 25 and the amplifier 30, comprises four elements: (1) loop noise due to the area 35 enclosed by the lead wires 10 and 15, (2) electrostatic noise picked up by the lead wires 10 and 15, (3) microphonic noise due to variable capacitance between the lead wires, bending of and vibrations of the lead wires, and (4) triboelectric noise due to static charge pickup as a result of mechanical rubbing of a conductor over an insulator when the wire is bent.
Because the amplifier 40 amplifies the differential signals detected by the EEG probes 20 and 25, respectively, by approximately one million, and because the amplitude of the signals detected by the EEG probes 20 and 25 is similar to that of the noise due to the lead wires 10 and 15, it is essential that the lead wires introduce little if any noise into the measurement system. Previous attempts at minimizing the noise introduced into the measurement system by the lead wires between EEG probes and an amplifier included using an expensive low pass filter 45 to attenuate the noise due to the lead wires. This approach is only effective for noise having a frequency greater than the frequency of the signal sought to be detected. For example in EEG systems, use of such an expensive low pass filter can be effectively used to attenuate noise having a frequency greater than, 10 Hz.
Another prior attempt at minimizing the noise due to EEG lead wires included using microphone cables for each of the lead wires 10 and 15. Typical systems using such cables, however, terminate the microphone cable shielding at, for example, the metal frame of a bed upon which the subject to be analyzed was resting. As a result, about 3 to 4 feet of unshielded lead wiring normally is used between the position at which the microphone shielding is attached to the metal frame and the EEG probes. In addition to being susceptible to electrostatic noise, this approach does not address either the noise due to the loop area 35 or the triboelectric effect noise.
Twisting the lead wires 10 and 15 into a twisted pair configuration minimizes the noise generated due to the loop area enclosed by the lead wires, but, does not address the noise due to triboelectric effects.
Previous attempts to minimize the noise due to the lead wires 10 and 15 connected between the EEG probes 20 and 25 and the amplifier 40, at best, addressed only one or two of the sources of noise; and, thus, are not satisfactory for reducing the noise due to the lead wires connected between EEG probes and associated amplifier.